Recycled fibre material, such as old corrugated containerboard (OCC), is commonly used as raw material for paper or board. The recycled fibre material comprises in addition to the fibres a number of other substances. Particulate foreign material is separated from the pulp in the pulper or at the screening. Some substances are naturally retained on the fibres and do not disturb the process. Other substances, such as stickies, may be separated from the pulp at the screening and removed from the process.
Typically recycled fibre material comprises starch, which has low molecular weight. This starch originates from the surface sizing of the paper or board, and it retains poorly on the fibres as it typically has no charge at all or it has a slightly anionic charge. Due to its small size it is not effectively separated at the screening either. Thus the low molecular weight starch remains in the water circulation of the pulping and papermaking processes or it is removed together with the screen effluent to the wastewater treatment. In the water circulation the starch increases the risk for microbial growth, as it is suitable nutritive substance for various microbes. Microbes may affect both the functioning of the chemistry of papermaking and/or the quality of the end product. High microbial activity can lower the pH and have a marked effect on wet-end chemistry. High microbial activity can also create strong odours that may be a nuisance or even a danger to operating personnel, and also destructive for product quality in packaging grades. Slime formation, biofilms, on the surfaces of tanks and machine frames leads to paper defects, such as spots and holes, or web breaks when slime lumps are sloughing off.
In the wastewater treatment the low molecular weight starch increases the COD load of the water to be treated, i.e. it increases the wastewater treatment costs.
The amount of low molecular weight starch in the recycled fibre material may be relatively high, for example 1-3% of the total weight of the recycled fibre. When the starch is lost to the water circulation during pulp preparation process the yield of the process is naturally decreasing. Therefore a process that would prevent the starch enrichment to the water circulation and assist its retention to the recycled fibres would provide numerous advantages.